We avail ourselves of this opportunity to make mention of the basic defense gearing for DK tanking. If you are interest in this information, you can follow with us to take a quick look at this in depth.
   You want to respec to a tank build and head straight into tanking 5-mans when you're at level 80. Get 540 defense is a very definite step you should take while you want to be the best tank. This will make immune to critical strikes from level 83 mobs, making it much easier to heal you in pretty much every situation, and it's really a prerequisite for serious tanking before any other stat.
When it comes to getting maximum defense, Death Knights are at a bit of a disadvantage. We don't get to wear shields, block rating gear is right out useless for us, and two-handed weapons aren't itemized for defense. Still, it's possible to hit that magic number, even without dual wielding tanking weapons.
There are a variety of defensive options for Death Knights that require a minimum of dungeon grinding to get. By picking and choosing, you should be able to get that 540 defense skill cap and hopefully then be able to tank a few of the more difficult normal and beginning heroic dungeons on your own.
As a reminder, defense skill is not the same as defense rating. Rather, you need defense rating to get defense skill. Specifically, you will need approximately 690 Defense Rating to gain the 540 defense skill you need to gain critical strike immunity against level 83 mobs.
Questing in Storm Peaks, you should stumble upon the Crown of the Hyldnir in the middle of the massive quest line started by They Took Our Men at K3. While 37 defense rating seems a little low, the helm does have a handy yellow gem slot, perfect for a Thick Autumn's Glow or a Thick Sun Crystal, bumping up that defense rating by a handy amount.
You may certainly want to skip the hassle with gem slot and just do some questing in Icecrown. By helping the Argent Crusade, you'll eventually get The Crusader's Resolution as a quest reward, a helm that not only beats out a gemmed Crown of the Hyldnir in defense rating and stamina, but also comes with some expertise, perfect for bumping up your threat.
You must take notice of the Helm of the Ley-Guardian from normal Oculus if you are doing some dungeon running. Not only does it beat out both of the above helmets on stamina and defense rating but it has a red gem slot which might look good with a Champion's Monarch Topaz in it. Really, if you have a chance to run the Oculus, just take it and pray for this helm to drop.
Make sure that you are able to grab revered with the Argent Crusade and enchant it with the Arcanum of the Stalwart protector to take care of a good part of your defense needs right there whichever helm you choose.
On shoulder needs you will have to look for a non-dungeon option for your defense rating, considering find a friendly Blacksmith to make you Tempered Saronite Shoulders. They're only level 76, but they have a nice bit of strength and stamina to go with their defense rating.
The Crusader's Square Pauldrons have the most armor of these three shoulder choice and some nice dodge rating so you must pay more attention to them. That's just perfect for keeping Bone Shield up a little while longer which they drop off trash in the Culling of Stratholme, but are bind on equip, so you may find them on the Auction House as well.